Chitosan Polymer Functionalized-Activated Carbon/Montmorillonite Composite for the Potential Removal of Lead Ions from Wastewater

A simple approach for synthesizing a highly adsorbent composite was described for the uptake of heavy metal ions from wastewater. A simple approach for synthesizing a highly adsorbent composite was also described for the elimination of heavy metal ions from contaminated water. The nanocomposite was synthesized via a polymer grafting of chitosan on the activated carbon surface, followed by a stacking process with the layers of montmorillonite clay. The spectroscopic analyses were exploited to confirm the composite structure of the prepared materials. Various adsorption parameters, such as pH, initial concentration, and adsorption time, were assessed. The results showed that the adsorption capacity of the composite for Pb2+ ions increased as the pH increased until it reached pH 5.5. The maximum adsorption capacity was observed at an initial Pb2+ level of 20 mg/L and a contact time of 150 min. Kinetic models were evaluated, and the pseudo second-order model showed the best match. The adsorption isotherm data were processed by fitting the model with different isotherm behaviors, and the Langmuir isotherm was found to be the most suitable for the system. The maximum adsorption capacity for Pb2+ ion on the MMT/CS/AC composite was found to be 50 mg/g at pH 5.5. Furthermore, the composite maintained a high adsorption capability of 85% for five adsorption–desorption cycles. Overall, this composite is envisioned as an addition to the market of wastewater remediation technology due to its chemical structure, which provides influential functional groups for wastewater treatment.


Introduction
Water treatment is a crucial process that ensures the safety and quality of the water supply. One of the most effective approaches for the uptake of organic compounds from water is adsorption. Adsorption, a cost-effective and environmentally friendly approach for water treatment, is widely used by many municipalities and industries [1,2]. Heavy metal removal from wastewater is receiving considerable focus at the moment because it represents one of the most important environmental challenges worldwide. The adsorption process is widely used for this purpose, and various materials have been efficiently introduced as adsorbents. However, the removal efficiency of the adsorbent depends on many parameters, including adsorbent crystallinity, degree of dispersion, and adsorbent functional groups involved in the chemical structure. Thus, introducing advanced adsorbents with a high removal efficiency of heavy metals is very important.
Clay minerals are paying a great deal of attention as potential adsorbents because they possess a highly ordered layered structure, a high ion exchange property, a low cost, and an increased abundance in nature. Montmorillonite (MMT) is a natural clay with a Milli-Q, Bay City, MI, USA) was utilized to prepare all the investigated solutions. All the materials employed in the work were employed as received without additional treatments.

Delamination of AC Layers
A certain quantity of AC was introduced to deionized water. The resultant suspension was submitted to sonication. The sonication process continued for an hour without controlling the medium temperature. After a sedimentation process of 24 h, the portion of the supernatant was separated. The remaining sediments of AC were oven-dried and weighed. The amount of AC in the supernatant portion was estimated from the difference of the starting AC weight and the sedimented AC weight.

Chitosan Polymer/Activated Carbon Composite Formation
CS/AC nanocomposite was prepared as follows. Firstly, a CS polymer solution was fabricated by dispersing 0.1 g of CS in 100 mL of 0.1 M CH 3 COOH. The obtained suspension was agitated overnight to completely disperse the chitosan. Partially protonated chitosan was synthesized by setting the pH at 6.0, utilizing 0.1 M KOH [23]. 1 g of dispersed AC was the introduced to the CS solution. The blend was submitted to sonication for 1 h using a Branson ultrasonic bath, and then stirred overnight at 80 • C to graft CS on the surface of the AC. The obtained composite was separated by a centrifuging process at 10,000 rpm utilizing a digitally operated high-speed centrifuge (Model TGL-16G), and then rinsed many times with aqueous acetic acid (pH 6) to remove unbonded CS moieties. A small portion of the formed composite was vacuum-dried for analysis.

Composite Formation of Montmorillonite/Polymer Functionalized-Activated Carbon
MMT layers were exfoliated as follows. As a first step, pristine MMT (5 g) was introduced to 100 mL of DI water. The suspension was shaken for 24 h, and then large aggregated and undispersed MMT particles were removed from the dispersion medium by filtration [24,25]. The weight of dispersed MMT remaining in the filtrate was estimated from the difference in weight between the starting and the undispersed. Subsequently, 1 g of dispersed MMT was poured to deionized water, and the final volume was augmented to 100 mL by deionized water. The suspension was then submitted to sonication for 1 h using a Branson ultrasonic bath. At this stage, 1 g of CS/AC composite was introduced to the MMT medium, and the resultant blend was additionally sonicated for 1 h. Finally, the obtained material was separated by centrifugation and vacuum, and labeled MMT/CS/AC [26].

Adsorption Assessment
The uptake of Pb 2+ ion from an aqueous solution using the prepared composite was conducted in a batch experiment. A 50 mL Pb 2+ ion (20 mg/L) was typically introduced to 10 mg of composite at a pH of 3-7 at a constant shaking time of 150 min. A series of studies were performed at various periods (5-420 min) at constant Pb 2+ ions level (20 mg/L), pH (5.5), dose (10 mg), and temperature (25 • C). The Pb 2+ ion concentration varies from 5-20 mg/L for isotherm study, whereas the other parameters were constant. Utilizing 0.1 mol/L NaOH or 0.1 mol/L HCl, the pH of each solution was adapted to the necessary values. The adsorption efficiency (percentage) and the quantity of Pb 2+ ion removed were evaluated by Equations (1) and (2): where, respectively, C o (mg/L) and C e (mg/L) are the original and the equilibrium concentration of Pb 2+ ion, m (g) is the sorbent mass of sorbent, and V (L) is the Pb 2+ ions' volume.

Desorption Studies
10 mg of the MMT/CS/AC composite was introduced to 25 mL of Pb 2+ ions solution for the desorption experiment. The resultant mixture was then shaken for 120 min at 100 rpm. The MMT/CS/AC composite that had been covered with Pb 2+ ions was then filtered. After that, the adsorbent was added to 25 mL of HCl (0.1 M) and agitated continuously for 120 min. The solution and adsorbent were separated, and the adsorbent was then used once more.

Characterization
Fourier-transform infrared spectra (FT-IR) were detected on a Perkin-Elmer FT-IR spectrophotometer. The IR spectra were collected by accumulating sixteen scans at 4 cm −1 resolutions. XRD was used to verify the amorphous/crystalline structure of the formed composites. The X-ray spectra were measured on a Shimadzu XRD-7000. Atomic absorption spectrometer (Thermo Scientific, Waltham, MA, USA) was used to determine the concentrations of lead ions. A digitally operated high-speed centrifuge was used to collect the prepared materials. Figure 1 Summary of the synthetic route used to synthesize the MMT/CS/AC composite. As seen, AC was grafted with CS polymer through a columbic interaction between the partially protonated amine groups ( + NH 3 ) of CS and the polyanionic functional groups (hydroxyl and carboxyl) of AC [20]. Subsequently, CS functionalized-AC electrostatically interacted with MMT layers, forming the MMT/CS/AC composite.  The amorphous/crystalline structure of AC was confirmed by X-ray analysis. Figure 2 displays the XRD spectra of AC prior to and after delamination. As seen from the spectrum of pristine AC (black spectrum), sharp peaks characteristic of a crystalline structure and aligned layers were observed at 2θ of 25 and 43 [27,28]. As a result of the sonication process of AC, a predominantly amorphous structure was obtained (gray spectrum), suggesting a successful delamination process of the AC layers [29]. This result is beneficial to the elimination behavior of AC. As a result of the grafting of AC with CS, the dispersibility of AC in aqueous media was improved. As seen from the photo images of Figure 3, AC became more attracted to water molecules (capable to create H-bonds) after grafting with CS (Figure 3a). At the same time, the as-received AC showed a fast The amorphous/crystalline structure of AC was confirmed by X-ray analysis. Figure 2 displays the XRD spectra of AC prior to and after delamination. As seen from the spectrum of pristine AC (black spectrum), sharp peaks characteristic of a crystalline structure and aligned layers were observed at 2θ of 25 and 43 [27,28]. As a result of the sonication process of AC, a predominantly amorphous structure was obtained (gray spectrum), suggesting a successful delamination process of the AC layers [29]. This result is beneficial to the elimination behavior of AC. As a result of the grafting of AC with CS, the dispersibility of AC in aqueous media was improved. As seen from the photo images of Figure 3, AC became more attracted to water molecules (capable to create H-bonds) after grafting with CS ( Figure 3a). At the same time, the as-received AC showed a fast sedimentation rate ( Figure 3b). This result indicated that CS has worked as a dispersing agent, providing AC with other ionic functional moieties on its surface, which is advantageous to the adsorption performance.

Result and Discussion
displays the XRD spectra of AC prior to and after delamination. As seen from the spectrum of pristine AC (black spectrum), sharp peaks characteristic of a crystalline structure and aligned layers were observed at 2θ of 25 and 43 [27,28]. As a result of the sonication process of AC, a predominantly amorphous structure was obtained (gray spectrum), suggesting a successful delamination process of the AC layers [29]. This result is beneficial to the elimination behavior of AC. As a result of the grafting of AC with CS, the dispersibility of AC in aqueous media was improved. As seen from the photo images of Figure 3, AC became more attracted to water molecules (capable to create H-bonds) after grafting with CS (Figure 3a). At the same time, the as-received AC showed a fast sedimentation rate (Figure 3b). This result indicated that CS has worked as a dispersing agent, providing AC with other ionic functional moieties on its surface, which is advantageous to the adsorption performance.   The CS/AC composite structure was confirmed by FTIR analysis. Figure 4a shows the FTIR spectrum of AC. The main absorption peaks of AC were noted at 3600, 1650, and 1000 cm −1 , which correspond to free OH, C=C and C-O groups, respectively [30]. The weak absorption bands recorded at 2100-2000 cm −1 were ascribing to bending modes of C-H groups [31]. The CS/AC composite structure was confirmed by FTIR analysis. Figure 4a shows the FTIR spectrum of AC. The main absorption peaks of AC were noted at 3600, 1650, and 1000 cm −1 , which correspond to free OH, C=C and C-O groups, respectively [30]. The weak absorption bands recorded at 2100-2000 cm −1 were ascribing to bending modes of C-H groups [31]. The CS/AC composite structure was confirmed by FTIR analysis. Figure 4a shows the FTIR spectrum of AC. The main absorption peaks of AC were noted at 3600, 1650, and 1000 cm −1 , which correspond to free OH, C=C and C-O groups, respectively [30]. The weak absorption bands recorded at 2100-2000 cm −1 were ascribing to bending modes of C-H groups [31].     [32,33]. The absorption bands observed at 2100-2000, 1600, 1440-1430, and 1395 cm −1 are related to the bending vibration modes of CH, NH 2 scissoring, CH 2 , and C-O-H groups, respectively. The predominant adsorption bands recorded at 1065-999 cm −1 are assigned to the stretching vibration modes of C-N groups overlapped with the stretching modes of the C-O-C group [34,35]. As a result of the grafting of CS on the AC surface, a shift in the absorption band of the N-H group to a lower wavenumber was observed (see Figure 4c). In addition, the band intensity of the free N-H group was decreased and broadened. Moreover, the IR spectrum of the composite showed the absorption bands of both CS and AC. These results suggested a successful interaction between CS and AC.
The composite structure of MMT/CS-functionalized AC was verified by XRD assessment. Figure 5a shows the X-ray diffraction pattern of MMT. Specifically, the characteristic (001) reflection of MMT was observed at 2θ = 7.4 • , from which the basal spacing d 001 calculated by Bragg's law was 1.2 nm [36]. The peaks recorded at 2θ of 20.0 • , 28.  [37]. Figure 5b shows the X-ray diffraction pattern of MMT interacting with CS-functionalized AC. As seen, the characteristic (001) reflection of MMT almost disappeared, in addition to a decrease in the intensities of the other MMT reflections. A broad diffraction peak at 2θ of 24.5 • newly emerged as an indication of the presence of AC moieties. These results suggested a successful exfoliation process of MMT layers and a good restacking with CS-functionalized AC [38].  Figure 5b shows the X-ray diffraction pattern of MMT interacting with CS-functionalized AC. As seen, the characteristic (001) reflection of MMT almost disappeared, in addition to a decrease in the intensities of the other MMT reflections. A broad diffraction peak at 2θ of 24.5° newly emerged as an indication of the presence of AC moieties. These results suggested a successful exfoliation process of MMT layers and a good restacking with CSfunctionalized AC [38].

Estimate the Optimal Parameters
The impact of different variables such as initial concentration, pH, and uptake time to optimize the removal conditions using the prepared materials was disclosed.

Effect of pH
The solution pH is a vital variable that affects the uptake process. In Figure 6a, the impact of the initial pH value on the uptake of Pb 2+ ion was shown. It should be noted that Pb 2+ ion adsorption capacity was attained at pH 5.5. At a low pH, the nanocomposite

Estimate the Optimal Parameters
The impact of different variables such as initial concentration, pH, and uptake time to optimize the removal conditions using the prepared materials was disclosed.

Effect of pH
The solution pH is a vital variable that affects the uptake process. In Figure 6a, the impact of the initial pH value on the uptake of Pb 2+ ion was shown. It should be noted that Pb 2+ ion adsorption capacity was attained at pH 5.5. At a low pH, the nanocomposite surface was loaded with a positive charge owing to the protonation of amine moieties. In addition, a significant electrostatic repulsion occurred between the composite surface and the Pb 2+ ion solution resulting in minimal desorption. However, as the solution's pH increases, the formation of metal hydroxides leads to a decrease in the efficiency of Pb 2+ ion removal [22]. The adsorption capacity for CS, MMT, AC, and MMT/CS/AC composite at pH 5.5 was 27, 31, 34.5, and 50 mg/g, respectively. In subsequent studies, pH 5.5 was thus accepted.

Impact of initial concentration
The efficacy of Pb 2+ ion adsorption on the synthesized MMT/CS/AC composite was explored in terms of the initial content of Pb 2+ ion (5-20 mg/L) at a contact time of 150 min, 10 mg dose, and pH 5.5 (Figure 6b). Pb 2+ ion removal capability increased from 11-50 mg/g as the concentration of Pb 2+ rose from 5 to 20 mg/L. This could be linked to the increased adsorption capacity acquired by raising the initial concentration of Pb 2+ ions due to the accessibility of unoccupied centers.

Impact of adsorption time
The influence of adsorption time on the Pb 2+ ion's removal capability was evaluated by adsorbent dose (10 mg) at pH 8.5. Obviously, for the fabricated MMT/CS/AC composite, the maximum adsorption capacity and 100% adsorption of Pb 2+ ion were obtained at an equilibrium time of 150 min (Figure 6c,d). When the contact duration was increased to 400 min, it was evident that the removal of Pb 2+ ions had not changed much from the measurements made at 150 min, as increasing the contact time resulted in the accumulation of the Pb 2+ ions, which hinders the diffusion of cumulative ions into the composite. This is the primary reason why there is no significant enhancement in adsorption as compared to that in 150 min.
the Pb 2+ ion solution resulting in minimal desorption. However, as the solution's pH increases, the formation of metal hydroxides leads to a decrease in the efficiency of Pb 2+ ion removal [22]. The adsorption capacity for CS, MMT, AC, and MMT/CS/AC composite at pH 5.5 was 27, 31, 34.5, and 50 mg/g, respectively. In subsequent studies, pH 5.5 was thus accepted.

Impact of initial concentration
The efficacy of Pb 2+ ion adsorption on the synthesized MMT/CS/AC composite was explored in terms of the initial content of Pb 2+ ion (5-20 mg/L) at a contact time of 150 min, 10 mg dose, and pH 5.5 (Figure 6b). Pb 2+ ion removal capability increased from 11-50 mg/g as the concentration of Pb 2+ rose from 5 to 20 mg/L. This could be linked to the increased adsorption capacity acquired by raising the initial concentration of Pb 2+ ions due to the accessibility of unoccupied centers.

Impact of adsorption time
The influence of adsorption time on the Pb 2+ ion's removal capability was evaluated by adsorbent dose (10 mg) at pH 8.5. Obviously, for the fabricated MMT/CS/AC composite, the maximum adsorption capacity and 100% adsorption of Pb 2+ ion were obtained at an equilibrium time of 150 min (Figure 6c,d). When the contact duration was increased to 400 min, it was evident that the removal of Pb 2+ ions had not changed much from the measurements made at 150 min, as increasing the contact time resulted in the accumulation of the Pb 2+ ions, which hinders the diffusion of cumulative ions into the

Kinetics Assessment
The kinetic model's evaluation was conducted to highlight the impact of the time of adsorption on the adsorption of Pb 2+ ions at pH 5.5 on the prepared composite. The pseudo first-order (Equation (3)), pseudo second-order (Equation (4)), and Elovich (Equation (5)) [39] were applied to obtain the relevant kinetics variables: where the pseudo first-order and pseudo second-order rate constants are k 1 (min −1 ) and k 2 (g mg −1 ·min −1 ). Q e and Q t (mg g −1 ) are the same as above. It is possible to calculate the constants α and β from the Temkin plot ( Figure 7 and Table 1).
where the pseudo first-order and pseudo second-order rate constants are k1 (min −1 ) and k2 (g mg −1 ·min −1 ). Qe and Qt (mg g −1 ) are the same as above. It is possible to calculate the constants α and β from the Temkin plot ( Figure 7 and Table 1).    The CS-Clay-AC composite's pseudo second-order correlation factor (r 2 ) was 0.9994, and the calculated adsorption capabilities (Qe) were 48.2 mg/g, in line with the measured ability of 50 mg/g. This information stated that Pb 2+ ion adsorption on the composite might function well on the pseudo second-order kinetics model (Figure 7b) [40]. The finding were not matched by the pseudo first-order and Elovich kinetics models (Figure 7a,c). In order to gain more understanding of the adsorption mechanism and rate regulatory processes affecting the adsorption kinetics, the kinetic data were additionally coupled to the intraparticle diffusion model. Equation (6) determines the model of intra-particle diffusion: where the intraparticle diffusion rate constants are the k id (mg/g min −1/2 ). The findings disclosed that two separate steps (Figure 7d) tacked the adsorption process. Additionally, the fact that the Q t vs. t 0.5 plot was unable to pass through the origin suggests that intraparticle diffusion was more than just a rate-control step, and that the border diffusion layer actually had an impact on the removal process [41].

Adsorption Isotherms
The isotherms of the adsorption portray how Pb 2+ ions distribute in the adsorption system at equilibrium between the bulk liquid and the solid state. The processing of the isothermal data by fitting it with distinct isotherm behaviors is a significant factor in exploring the suitable isotherm that can be used for design purposes. The isotherm findings were applied to Langmuir (Equation (7)), Freundlich (Equation (8)), and Temkin (Equation (9)) isotherms: C e Q e = 1 Q max C e + 1 Q max K L (8) where the amount of Pb 2+ ion adsorbed at equilibrium (mg/g) and the concentration at equilibrium (mg/L) are Q e and C e . K f (mg/g) and n are the Freundlich constants given respectively to the adsorption capacity and intensity. Q m is the highest adsorption amount (mg/g), and k L is the adsorption energy-related Langmuir constant (L/mg). A T is the equilibrium binding constant, (L/g), b T is the adsorption constant (J/mol K), and B is the heat of sorption-related constant (J/mol). The small correlation factor (r 2 ) of Freundlich and Temkin isotherms shows that they do not match both models, as shown in Figure 8 and Table 2. In contrast, the synthesized MMT/CS/AC composite's Pb 2+ ion adsorption data fit the Langmuir isotherm with the highest regression factor. Finally, the fitting of experimental data allows the ordering of Langmuir, Freundlich, and Temkin by the regression factor (r 2 ) of different isotherms. These findings show that the combination of the individual constituents of the composite favors the Pb 2+ ion's adsorption in a monolayer coverage [41]. It is, therefore, practical to use MMT/CS/AC composite as adsorbents for the cleaning process with a notable tailored structure.

Reusability Assessment MMT/CS/AC Composite
For sophisticated adsorbent materials, recycling, and reusability are significant considerations. According to the pH survey, removing Pb 2+ ions on the composite was low at smaller pH values, meaning that the removed Pb 2+ ions can be detached in an acidic medium from the synthesized composite. An acidic solution of HCl (0.1 mol/L) was used in the desorption process because of the petite sizes of the chloride's ions compared to other counter ions. As shown in Figure 9, a cycling experiment was repeated five times using the same Pb 2+ -loaded MMT/CS/AC composite. The MMT/CS/AC composite maintained a high capacity of adsorption throughout the fifth adsorption-desorption cycle.
and Temkin isotherms shows that they do not match both models, as shown in Figure 8 and Table 2. In contrast, the synthesized MMT/CS/AC composite's Pb 2+ ion adsorption data fit the Langmuir isotherm with the highest regression factor. Finally, the fitting of experimental data allows the ordering of Langmuir, Freundlich, and Temkin by the regression factor (r 2 ) of different isotherms. These findings show that the combination of the individual constituents of the composite favors the Pb 2+ ion's adsorption in a monolayer coverage [41]. It is, therefore, practical to use MMT/CS/AC composite as adsorbents for the cleaning process with a notable tailored structure.

Reusability Assessment MMT/CS/AC Composite
For sophisticated adsorbent materials, recycling, and reusability are significant considerations. According to the pH survey, removing Pb 2+ ions on the composite was low at smaller pH values, meaning that the removed Pb 2+ ions can be detached in an acidic medium from the synthesized composite. An acidic solution of HCl (0.1 mol/L) was used in the desorption process because of the petite sizes of the chloride's ions compared to other counter ions. As shown in Figure 9, a cycling experiment was repeated five times using the same Pb 2+ -loaded MMT/CS/AC composite. The MMT/CS/AC composite maintained a high capacity of adsorption throughout the fifth adsorption-desorption cycle.

Adsorption Mechanism
The removal process of Pb ions by the synthesized MMT/CS/AC composite was shown in Figure 10. As demonstrated, both the functional moieties of CS (amine moieties)

Adsorption Mechanism
The removal process of Pb ions by the synthesized MMT/CS/AC composite was shown in Figure 10. As demonstrated, both the functional moieties of CS (amine moieties), AC (carbonyl moieties), and the negative layers of MMT efficiently contributed to both the capture and the uptake of positive Pb ions from wastewater, showing a high adsorption capability. The removal of Pb ions from contaminated water thus correlated to a chelation process [42]. The removal capacity values obtained in this study exceeded the reported values of various adsorbents for removing Pb(II) ions (Table 3)

Conclusions
In conclusion, the simple approach described in this a highly adsorbent composite that efficiently removed hea A multi-component composite based on CS polymer, A was successfully formulated via a grafting/delaminatio potential composite for the uptake of heavy metal ions composite, which consisted of a chitosan-polymer grafting and layers of montmorillonite clay (MMT/CS/AC), exhib of 50 mg/g for Pb 2+ ions at pH 5.5. The composite al maintained a high adsorption capability of 85% for five ad research highlights the significant of this composite as a treatment and offers a promising addition to the mar technology.   A commercial activated carbon adsorbent (CGAC) C o : 50-500 mg/L; pH: 7; T: 298 K; m: 2 g 180 20.3 [43] Citric acid modified pine sawdust (CA-PS) C o : 5-220 mg/L; pH: 5; T: 298 K; m: 0.2 g 120 16.19 [44] Pb 2+ adsorption by a compost C o : 2-50 mg/L; pH: 5; T: 298 K; 3.0 g/L adsorbent 1440 21.45 [45] Biochar-supported graphene oxide composite for removal of lead ion pH: 5; T: 298 K 900 26.1 [46] Algal biomass Sargassum glaucescens C o : 0-100 mg/L, 2.0 g/L adsorbent 120 45.8 [47] Raw agave bagasse

Conclusions
In conclusion, the simple approach described in this study successfully synthesized a highly adsorbent composite that efficiently removed heavy metal ions from wastewater. A multi-component composite based on CS polymer, AC nanomaterial, and MMT clay was successfully formulated via a grafting/delamination/restacking technique to be a potential composite for the uptake of heavy metal ions from contaminated water. The composite, which consisted of a chitosan-polymer grafting on the activated carbon surface and layers of montmorillonite clay (MMT/CS/AC), exhibited the highest uptake capacity of 50 mg/g for Pb 2+ ions at pH 5.5. The composite also showed good stability and maintained a high adsorption capability of 85% for five adsorption-desorption cycles. This research highlights the significant of this composite as a potent solution for wastewater treatment and offers a promising addition to the market of wastewater remediation technology.